netdev - Re: [PATCH v2 net 01/15] af_unix: Set sk->sk_state under unix_state

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Date: Wed, 26 Jun 2024 14:56:55 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <mhal@...x.co>
CC: <cong.wang@...edance.com>, <davem@...emloft.net>, <edumazet@...gle.com>,
	<kuba@...nel.org>, <kuni1840@...il.com>, <kuniyu@...zon.com>,
	<netdev@...r.kernel.org>, <pabeni@...hat.com>
Subject: Re: [PATCH v2 net 01/15] af_unix: Set sk->sk_state under unix_state_lock() for truly disconencted peer.

From: Michal Luczaj <mhal@...x.co>
Date: Wed, 26 Jun 2024 12:48:27 +0200
> On 6/23/24 07:19, Kuniyuki Iwashima wrote:
> > From: Michal Luczaj <mhal@...x.co>
> > Date: Sun, 23 Jun 2024 00:43:27 +0200
> >> I gotta ask, is there a reason for unlinking an already consumed
> >> ('consumed' as in 'unix_skb_len(skb) == 0') skb so late, in manage_oob()?
> >> IOW, can't it be unlinked immediately once it's consumed in
> >> unix_stream_recv_urg()? I suppose that would simplify things.
> > 
> > I also thought that before, but we can't do that.
> > 
> > Even after reading OOB data, we need to remember the position
> > and break recv() at that point.  That's why the skb is unlinked
> > in manage_oob() rather than unix_stream_recv_urg().
> 
> Ahh, I see. Thanks for explaining.
> 
> One more thing about unix sockmap. AF_UNIX SOCK_DGRAM supports 0-length
> packets. But sockmap doesn't handle that; once a 0-length skb/msg is in the
> psock queue, unix_bpf_recvmsg() starts throwing -EFAULT. Sockmap'ed AF_INET
> SOCK_DGRAM does the same, so is this a bug or a feature?

I guess it's kind of safeguard.

The retval 0 has special meaning for SOCK_STREAM as EOF/shutdown().
If we bypass 0-byte dgram to SOCK_STREAM sk, the application will be
confused as if the original peer has disconnected.

At least, -EFAULT avoids such confusion so that can only the true
peer trigger 0-byte via the saved ->recvmsg().

So, the requirement would be similar to scm handling, we need to
recognize the sockmap verdict and destination to support full
features.